Centrifugal compressors are utilized in a number of industrial applications. The major components of a centrifugal compressor are the impeller which is driven by a power source, typically an electric motor. The impeller rotates within an inner annular region of a hub plate and adjacent to a shroud. The impeller is a rotating bladed element that draws the fluid to be compressed through the shroud and redirects the flow at high velocity and therefore kinetic energy in a direction that is generally radial to the direction of rotation of the impeller. A diffuser is located downstream of the impeller within a diffuser passage area defined between the hub plate and an outer portion of the shroud to recover the pressure in the gas by decreasing the velocity of the fluid to be compressed. The resulting pressurized fluid is directed towards an outlet of the compressor.
In vaneless diffusers, the diffuser passage area between the hub plate and the outer portion of the shroud is ever increasing to recover the pressure. In vane-type diffusers, blades are connected to the hub plate or the outer portion of the shroud in the diffuser passage area. The blades can have a constant transverse cross-section as viewed from hub plate to shroud. In vane-type diffusers, known as airfoil diffusers, the vanes have an airfoil section rather than a constant transverse cross-section.
The power that is required to drive such a centrifugal compressor can represent a considerable portion of the running cost of the plant in which the centrifugal compressor is employed. For example, in an air separation plant, most of the costs involved in operating the plant are electrical power costs used in compressing the air. Compressors employed in such applications as air separation, but other applications as well, require a wide operating range. For example, in an air separation plant, it is necessary to be able to turn down the production and to raise the production. This variable operation can be driven by demand or local electrical power costs which will vary depending on the time of day. However, given the cost of electrical power, it is also necessary that the wide operating range be accompanied by compressor efficiency over the operating range.
In an attempt to increase the operating range while retaining efficiency, it is possible to alter impeller design and diffuser design. With respect to impeller design, however, the actual design employed is constrained by the mechanical arrangement of the compressor and the resulting flow conditions, for instance specific speeds. These arrangements, lead to a predetermination of many of the impeller characteristics, for instance, the design of the impeller shroud and inducer arrangements, axial length and therefore, meridional profile and the use of three-dimensional aerodynamic configurations, namely aerodynamic sweep and lean and the use of splitter blades. Typically, however, the most commonly used impeller characteristic is blade backsweep at the impeller exit. This gives the centrifugal stage a rising pressure characteristic with decreased flow rates which increases the stability of the stage. Furthermore, compared to a radial bladed impeller designed at the same rotation speed and pressure ratio, a backswept impeller has lower blade pressure loading as compared to a radial bladed impeller design, increased impeller reaction and increased loss free energy transfer (Coriolis acceleration) to the fluid.
The diffuser design is less constrained than the impeller. The geometrical constraint for the diffuser design being the size of the volute and collector for overhung stages, or return channel in the case of beam type stages. Vaneless diffusers are able to provide the centrifugal compressor stage with large operating ranges at moderate pressure recovery levels and at moderate efficiencies. Vane-type diffusers, on the other hand, have a higher efficiency level but at reduced ranges. In an attempt to increase the range of operation, U.S. Pat. No. 2,372,880 provides a vane-type diffuser having blades without an airfoil transverse cross-section but with a twist built into the blades to change the throat area and thereby to increase the operating range of the compressor. The resulting diffuser is a high solidity diffuser or in other words geometrically incorporates a ratio, calculated by dividing a distance measured between the leading and trailing edges of the blades by the circumferential spacing between leading edges of adjacent blades, that is greater than 1.0.
Low solidity diffusers, that is airfoil diffusers with a solidity value of less than 1.00 are characterized by the absence of a geometrical throat in the diffuser passage and have proven to possess a large flow range, similar to vaneless diffusers, but at increased pressure recovery levels over vaneless diffusers. The increased range in operation, however, has been found to be at the expense of efficiency compared to high solidity diffusers. At the other extreme, high solidity diffusers have been constructed, that while more efficient, do not possess the operating range of low solidity diffusers.
As will be discussed, in the present invention, in one aspect, provides an airfoil diffuser in which the diffuser blades are fabricated with a twisted configuration that produce a low solidity value at the hub plate and a high solidity value at the shroud with the result that the diffuser imparts to this centrifugal compressor not only a wider operating range but also high efficiency over the wide operating range as compared to the prior art.